For certain uses requiring fibrous masses with a certain degree of porosity, for example in the manufacture of filters, the porosity required is in reality that which can be obtained with non-woven fabrics or rovings, even carded rovings, i.e. a porosity that is, so to speak, `inter-filamentary`, created by the interstices formed between the individual fibres that make up the non-woven fabric.
Now a new type of porous structure fiber has been found which is able to replace the porous fibrous masses so far used in all kinds of applications, with considerable advantages in terms of the consumption of materials and of cost.